Nanoelectromechanical systems (NEMS) are a developing field of nanotechnology. NEMSs include nanotube devices that depend on mechanical movement. Nanotubes may also be referred to as nanorods, nanofibers, and nanowhiskers. A tunable radio frequency (RF) filter is an example of a NEMS.
Some frequency-tunable resonators use a capacitive nature for actuation/transduction. These frequency-tunable resonators typically include massively parallel arrays. The number of tubes or fibers in such arrays is on the order of 10,000-100,000.
The most straightforward geometry for such arrays is shown in FIG. 1. This geometry will work if the tubes are spaced far enough apart that the side electrodes screen the Coulomb interaction between the tubes.
Moving the tubes closer together causes the Coulomb interactions between the tubes to exert more of an effect on the tubes. This interaction is useful in the sense that collective resonances may be used. The drawback to this arrangement, however, is that the charge distribution on the tubes/fibers in the array will not be homogenous. More charge will accumulate on the tubes at the edges of the array than on the ones in the interior of the array.
FIG. 1 illustrates a traditional linear array resonator 100. The incoming RF signal on the gate 110 actuates the CNTs/CNFs (carbon nanotubes/carbon nanofibers) 150 when the source bias voltage VS is such that the system is on resonance with the incoming signal frequency. The bias voltage is the difference between Vg and Vs. The source need not be electrically grounded; it may have a potential Vs and Vg may be offset with the same potential.
Through capacitive transduction, the signal is transferred to the drain electrode 120 when the system is on resonance. Hence the device acts as a tunable filter. Because the tubes are interacting, the charge distribution on the tubes 150 along the array direction (x-direction) is not uniform. The typical distribution is shown as Q in the inset. This distribution may cause the tubes 150 close to the edges to be tuned more and lead to early snap-in.
Due to the disparity of charge, when a sufficient voltage is applied, the tubes 150 at the edges will be more attracted to the electrodes, and may snap into contact as shown in FIG. 2a. Once the two outermost tubes 150 have snapped into contact a DC-path may be available for a current and destruction of the device may result.
While it may be possible to prevent the development of a DC path, for example, by a dielectric layer on the electrode, this presents another problem. As soon as the outermost tubes 150 have snapped to contact, the charge distribution on the remaining tubes 150 in the array is redistributed to once again be focused around the edges. Hence, more tubes 150 may snap into contact (as shown in FIG. 2b). This will lead to a cascade effect where the whole array eventually snaps into contact.
FIG. 2b illustrates the early snap-in cascade phenomena. As shown in FIG. 2 the tubes 150 at the end of the array carry more charge. These tubes 150 are more attracted to the drain electrode 120 than the tubes 150 in the center. Thus they will snap into contact before the inner nanotubes 150. Once the outermost tubes 150 have snapped-in, as shown in FIG. 2a, the charge redistributes such that the two outermost tubes 150 that have not snapped into contact will experience a larger force. These will then snap into contact as well, as shown in FIG. 2b. This creates a cascade effect which may result in all of the tubes 150 snapping into contact.
Because the main transduction comes from the members in the interior of the array the snap-in cascade effect results in the full frequency tuning range of the device being unavailable.
Reference may be made to PCT/SE02/00853 for a description of a nanotube relay device where a nanotube flexes to close an electrical circuit.
PCT/SE05/00691 describes an arranging of several nano-relays that exploit the electromechanical resonance of the system to function as a filter. See U.S. Pat. No. 6,737,939 and U.S. Pat. No. 7,301,191 for further examples.
Some prior solutions to the snap-in problem have been presented, for instance, having a non-uniform spacing between the tubes, the inter-tube distance increasing near the edges, or using electrodes that curve away close to the edges. The drawback of the first solution is that interactions between members in the array become non-uniform and collective resonant modes are less likely to prevent disorder induced broadening of the resonance. Both alternative solutions also suffer from fabrication caveats because they require either patterning of tubes or the electrode geometry to be precisely controlled.
What is needed is a more practical solution that enhances the tuning range of resonators by solving the problem of charge accumulation at the nanotube array edges.